A methyl or ethyl ester of a higher fatty acid (e.g., oleic acid, stearic acid) that is contained in a fruit or seed of a plant in abundance is called a clean diesel fuel “a biodiesel oil” and has been focused as one of the next generation fuels. A biodiesel oil can be used as a fuel for existing diesel engines. A biodiesel oil produces an exhaust gas free of any sulfur component upon being combusted, and the amount of black smoke generated by the combustion is remarkably reduced compared to a conventional diesel fuel that is generated from petroleum. More importantly, CO2 produced by the combustion of a biodiesel oil is fixed again in a plant during the growth stage of the plant; in other words, a biodiesel oil is a zero CO2 emission fuel. If the cultivation, harvest and treatment of a crop containing a higher fatty acid in abundance and the production reaction, separation and purification of a biodiesel oil from the crop can be achieved at good efficiency, a clean energy of higher quality could be produced. For these reasons, an interest in biodiesel oils has been growing year by year, and the amount of biodiesel oils produced in the world is estimated to exceed 1,700,000 kl in 2005. While the amount of production of biodiesel oils in European countries where many diesel vehicles are used accounts for 99% of the total production amount, the amount of production of biodiesel oils in Japan accounts for only about 0.1%.
In European countries, a biodiesel oil is produced in large quantity by using a crude oil produced by expressing a crop such as soybean as a raw material. However, a “free fatty acid” contained in the crude oil becomes a stumbling block for the efficient production of a biodiesel oil. A crude oil contains an oil-and-fat (triglyceride; an ester between a higher fatty acid and glycerin) and a considerable amount of a free fatty acid (which is a higher fatty acid present in a free form, not in the form of an oil-and-fat). What we generally call “edible oil” is one produced by removing a free fatty acid from a crude oil. When an alcohol and an alkali hydroxide are added to an oil-and-fat, a transesterification occurs by the basic catalytic action of the alkali hydroxide to thereby produce a higher fatty acid ester (a biodiesel oil) and glycerin. However, when it is tried to synthesize a biodiesel oil by adding an alcohol and an alkali hydroxide to a plant-derived crude oil, a reaction between a free fatty acid with the alkali occurs preceding to the transesterification and, as a result, a soap and water are produced. Water deteriorates the catalytic action of an alkali remarkably. Further, a product may be contaminated by water produced during the reaction by the action of a soap that can act as a surfactant. Therefore, the presence of water makes the synthesis and separation of a biodiesel oil difficult. For the purpose of producing a biodiesel oil from a plant-derived crude oil in large quantity and at high efficiency, a free fatty acid is usually removed from the crude oil. However, the removal of a free fatty acid requires an enormous amount of energy. In recent years, a process has also been employed in which a free higher fatty acid contained in a crude oil is converted into a biodiesel oil by the esterification using an acid catalyst and the oil-and-fat remaining in the reaction system is then converted into a biodiesel oil in a conventional manner. However, since there is no solid acid catalyst which enables the efficient proceeding of the esterification, the reaction in the first stage of the process has no choice but to rely on a liquid acid such as sulfuric acid and hydrochloric acid. As mentioned above, when a liquid acid is used as a catalyst, an enormous amount of energy is necessary for separating a product from the catalyst after the reaction. Thus, for the production of a biodiesel oil, a large amount of energy is used for the separation and purification of the biodiesel oil, which accounts for 20 to 50% of the cost required for the mass production of a biodiesel oil.
On the other hand, the present inventors have found that an amorphous carbon having a sulfonate group introduced therein can act as a solid catalyst and already filed patent applications (Patent reference Nos. 1 and 2). Although these patent applications describe that the carbon can catalyzes the esterification between ethanol and acetic acid, it is not described that the carbon can catalyze the esterification between ethanol and a higher fatty acid.    [Patent reference No. 1] Japanese Patent Application Laid-open No. 2004-238311    [Patent reference No. 2] International Publication No. WO 2005/029508